This invention relates to a dredging helical cutter and a method for more efficiently dredging an underwater area. In the past, dredgers have had the general configuration shown in FIGS. 4 and 5, wherein on one end section of a hull 8, the base end portion of a rudder 9, which is swung vertically depending upon the depth of the bottom of the water, is attached to a transverse shaft 10. A suspension rope 11, suspended from a winch provided on the hull 8, is coupled to the point portion of the rudder 9 through a pulley provided on the point of a boom 12 fixed on the hull 8. The point portion of the rudder 9 is equipped with a cutter 13 which is rotated by a driving device. In the other end section of the hull 8, a first spud 14 and second spud 15 are provided. The first and second spuds 14 and 15, are elevatably movable and mutually spaced in the widthwise direction of the ship.
When the ground at the bottom of the water is to be excavated using this dredger, the first spud 14 is fixed to the ground 16. The first step of excavation is performed using the cutter 13, which is spaced R meters from the first spud 14, while turning the hull 8 and rudder 9 laterally in either direction. For example, after the rudder 9 is rotated downward a given angle (i.e., the angle corresponding to the distance through which the cutter 13 moves down, which is about one-half of the diameter of the cutter), the hull 8 and rudder 9 are moved laterally clockwise about the first spud 14. The second step of the excavation is performed while turning the hull 8 and rudder 9 counterclockwise. After the rudder 9 is rotated downward a given angle, the third step of the excavation is performed while again turning the hull 8 and rudder 9 clockwise.
After the repetitive turning excavation described above has reached a given depth, the cutter 13 is kept at the final right turn position, the rudder 9 is rotated upward, the second spud 15 is put through the ground 16, the first spud 14 is pulled out, the hull 8 and rudder 9 are turned counterclockwise about the second spud 15 up to the final left turn position, the first spud 14 is put through the ground 16 at position 14A, the second spud 16 is pulled out (through the foregoing series of operations, the hull 8 has moved and advanced in the forward direction of excavation), and a second repetitive turning excavation similar to the first one is performed. Then, a series of operations similar to that described above is repeated, and successively, the repetitive turning excavations are performed.
Conventional cutters used in the foregoing dredger are of the helical type, as shown in FIG. 6. Helical vanes 3 are fixed on a rotary body 2, which is fitted and secured to a driving shaft 1. A number of excavating picks 4 are attached to the periphery of each helical vane 3, and an inlet port 6 of an earth/sand suction tube 5 supported by a bearing 22 fitted on the driving shaft 1, is disposed opposite to the lower portion of the send-out end portion (i.e., the end portion of the vanes of the helical cutter toward which sand and dirt to be excavated is moved) of the helical vanes 3.
When the foregoing conventional helical cutter is used to dredge, earth and sand and the like, excavated by the picks 4, are conveyed by the helical vanes 3 toward the inlet port 6 while being guided along an excavated plane 18. However, a greater part of earth and sand flows out along a previously excavated, formed, inclined plane 19 after reaching the inlet-port side end portion, i.e., the terminated portion, of the excavated plane 18, thereby causing inefficiency in the suction of earth and sand by the earth/sand suction tube 5.